Scientists Throw Light on the Migration of Impurity Atoms in Tungsten Material

One segment—that is, the plasma facing material—of the vacuum vessel in the fusion experimental device and prospective fusion reactor comes into contact with plasma. Upon entry of the plasma ions into the material, namely, tungsten, those particles are modified into neutral atom and remain inside the material.

The simulation result is based upon the dynamic Monte Carlo method4). Inside the base material of the tungsten, the atoms of which structure is close to the crystal are not displayed, and the tungsten atoms near the crystal grain boundary, which are of out-of-crystal deformation, are displayed as yellow and blue-colored points. The tracks of the impurity atoms moving inside (in this example, helium atoms) are displayed in white. So to make the impurities moving at high speed visible to the human eye, in order to indicate the tracks of the impurity atoms we have made all tracks resemble a tail with a fixed length, and we are seeking to express that movement of clouds as “averaged behavior.” CREDIT: Dr. Atsushi M. Ito.

With respect to the atoms that form the material, the entered plasma ions turn out to be impurity atoms. Gradually, these impurity atoms move into the spaces in between the atoms that form the material. Ultimately, the impurity atoms get distributed inside the material. In contrast, a portion of the impurity atoms goes back to the surface and these atoms are once more emitted to the plasma. In order to confine the fusion plasma in a stable manner, it is highly significant to maintain a balance between the entry of the plasma ions into the material and the re-emission of the impurity atoms after moving from the inner side of the material. The path of movement of the impurity atoms inside the materials that have optimal crystal structure has been explained well in many studies.

Yet, actual materials possess polycrystalline structures and the paths of migration in grain boundary regions have not been elucidated so far. Moreover, in the case of a material that is constantly in contact with plasma, the crystal structure is disrupted because of the uncontrolled invasion of plasma ions. There has been no adequate analysis of the paths of migration of impurity atoms inside a material that has a disrupted crystal structure. Professor Atsushi Ito from the National Institutes of Natural Sciences (NIFS), and his team have been triumphant in developing a technique for performing quick and automatic exploration of migration paths in materials with arbitrary atomic geometry by using molecular dynamics and by carrying out parallel computations with a supercomputer.

Initially, they selected numerous tiny domains covering the entire material. Then, they applied molecular dynamics to calculate the paths of migration of impurity atoms inside each tiny domain. The computations for the small domains can be completed in a short period of time as the size of each domain is small and the number of atoms to be taken into account is very few. As the computations for every small domain can be made independently, the computations are simultaneously carried out by means of the NIFS supercomputer, the Plasma Simulator and the HELIOS supercomputer system at the Computational Simulation Centre of International Fusion Energy Research Centre (IFERC-CSC), Aomori, Japan.

As the Plasma Simulator enables the usage of 70,000 CPU cores, it can be used to perform parallel computations for more than 70,000 domains. The migration paths for the entire material can be acquired by integrating the computation outcomes from all the small domains. This parallelization technique of the super computer is different from the one that is normally used and is known as MPMD-type parallelization. A simulation technique that effectively applies MPMD-type parallelization has been put forward at NIFS. The Researchers have integrated the parallelization with new concepts related to automatization and have developed a high-speed automatic technique for searching the migration path. The new technique enables the path of migration of the impurity atoms to be easily searched in the case of actual materials including crystal grain boundaries or even materials with disordered crystal structure due to long-time exposure to plasma. Investigating the behavior of collective migration of impurity atoms inside material based upon information regarding this migration path, we can deepen our knowledge regarding the particle balance inside the plasma and the material. Thus improvements in plasma confinement are anticipated.

The outcomes of the research were reported at the 22nd International Conference on Plasma Surface Interaction (PSI 22) in May 2016, and will be published in the Nuclear Materials and Energy journal.

In fusion research, the interaction between material and plasma is analyzed as an objective of ensuring the long-standing durability and integrity of wall materials. In disciplines other than fusion, this interaction is analyzed for developing processing methods that involve active usage of plasma, such as processing of surface coating and semiconductors, among others. The automatic technique, described in this study, for analyzing the path of migration of impurity atoms in the material that comes into contact with the plasma is at present being considered for usage in material processing where this type of plasma is used. As the technique can be generally applied for analyzing migration of additive material atoms and impurity atoms, it can be expected to serve for a wide range of applications in different fields.

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